It is sometimes useful or advantageous to form, in a same wafer, a layer of a first crystalline material on a substrate of a second crystalline material that has a different nominal lattice parameters respectively, but while keeping the crystal structure of the second material relaxed or mostly relaxed and/or without an excessive number of crystallographic defects. For this purpose, it is known to insert a buffer layer between the substrate and the formed layer.
In this configuration, a “buffer layer” is understood to mean a transition layer that matches the lattice parameter of the formed layer with that of the substrate. For this purpose, such a buffer layer may have a composition that gradually varies through the thickness, the gradual variation of components of the buffer layer being directly associated with a gradual variation in its lattice parameter between the respective lattice parameters of the substrate and of the formed layer. The buffer layer may also have a more complex form, such as a variable-content compositional variation, an inversion in the sign of the content, or discontinuous step changes in composition.
The formation of such a variable composition generally takes a long time and is often complex to implement. Moreover, to minimize the density of crystallographic defects in the buffer layer, the thickness of a buffer layer is usually large, typically between one and several microns. The production of such a buffer layer therefore involves often lengthy, difficult and expensive processing.
A technique of relaxing elastic strains in a formed layer giving a similar result with substantially less processing required is taught by B. Höllander et al. in the document entitled “Strain relaxation of pseudomorphic Si1-xGex/Si(100) heterostructures after hydrogen or helium ion implantation for virtual substrate fabrication” (in Nuclear and Instruments and Methods in Physics Research B 175–177 (2001) 357–367). The process described relates to the relaxation of a strained SiGe layer in compression, with this layer being formed on an Si substrate.
The technique employed comprises the implantation of hydrogen or helium ions through the surface of the strained layer into the Si substrate to a defined depth. A crystalline perturbation created by the ion implantation and located in a thickness of the Si substrate lying between the implantation region and the SiGe layer causes, under a heat treatment, a certain relaxation of the SiGe layer.
A process is needed to readily relax the strain on a layer with efficiency of material and to transfer this layer to another substrate.